DE 195 16 071 A1 shows a sheet-fed printing machine having a sheet delivery mechanism for changing over from one stack to another while permitting continuous sheet feeding. In this case, a separating element is used which includes a shaft arranged outside the stacking region and one or more wedges, wherein the separating element provides a triangular space required for inserting the stack rake. The separating element lies on the top sheet of the main stack in the region of the front edge. The additional sheets received are simultaneously placed on the auxiliary stack now being formed above the separating element. The sheets accumulated above the separating element are released when the separating element is turned about the shaft and follows the main stack being lowered, wherein the separating element is subsequently pivoted out of the stacking region. This device can only be used with stack rakes because the wedges must contact the upper edge of the main stack through the intermediate spaces of the stack rake.
A sheet delivery mechanism with a device for changing over from one stack to another in a sheet-fed printing machine with continuous sheet feeding is also is known from DE 198 19 491 C1. This device is arranged upstream of a delivery stack (i.e. first stacking system) and functions as a switch, wherein the device selectively deposits the sheets on the delivery stack (first stacking system) or on a second stacking system arranged upstream of the first stacking system. During a nonstop stack changing mode, for example, the switch is activated in such a way that the sheets can be deposited on the second stacking system. In the meantime, the main stack of the first stacking system, for example, can be lowered and transported away from the sheet delivery mechanism, wherein a new stack board or stack rake is previously inserted, and wherein the sheets that were transported to the second stacking system are now fed to the delivery stack (first stacking system). The stack of the second stacking system can be exchanged in the meantime.
When changing a stack, however, it is disadvantageous that sheet material which is properly aligned change its position in the stack when the stack is changed. The lowering movement and/or air enclosed between the sheets and the resulting reduction in static friction, particularly in the upper region of the stack can cause the sheets to change their position in the stack.